The present invention relates to a concentrated dampening water composition useful for offset printing using a lithographic printing plate.
Lithographic printing technique makes the best use of the properties of water and an oil such that they are essentially incompatible with one another. The printing surface of a lithographic printing plate comprises areas which receive water and repel an oil ink and those which repel water and receive an oil ink, the former serving as non-image areas and the latter serving as image areas. The non-image areas become damp with dampening water used in lithographic printing which contains a desensitizing agent to thus enhance the difference in surface chemical properties between the image areas and the non-image areas and hence to increase both the ink repellency of the non-image areas and the ink receptivity of the image areas.
As such dampening water, there have generally been known conventionally aqueous solutions containing such inorganic substances as alkali metal salts or ammonium salt of bichromic acid, phosphoric acid or salts thereof such as ammonium salt, and such a colloidal substance as gum arabic or carboxymethyl cellulose (CMC).
However, it is difficult to uniformly dampen the non-image areas of lithographic printing plates with the dampening water containing such a desensitizing agent and for this reason, the resultant printed matters are sometimes contaminated and a substantial skill in controlling the feed rate of the dampening water is required.
To overcome such disadvantages, there has been proposed the Dahlgren dampening system in which an aqueous solution containing about 20 to 25% of isopropyl alcohol is used as dampening water. This method provides a variety of advantages concerning workability and accuracy of printed matters, such that the wettability of the non-image areas is improved, the amount of the dampening water can be reduced, it is easy to control the balance between feed rates of printing ink and dampening water, the amount of water emulsified into the printing ink is lowered and the transfer of printing ink to the blanket is improved.
However, isopropyl alcohol is apt to evaporate and, therefore, the use of a special device is required for keeping the concentration thereof constant. This is unfavorable from an economical point of view. Moreover, isopropyl alcohol is malodorous and is toxic and thus the use thereof is not favorable in view of the pollution of the working atmosphere.
In addition, even if the dampening water containing isopropyl alcohol is applied to offset printing in which a dampening molleton roller is commonly used, isopropyl alcohol evaporates from the roller surface and the printing plate surface. Therefore, it cannot show its own effects.
Moreover, the pollution with industrial waste has become a matter of great concern, the regulation with respect to discharge of chromium ions in waste water has become much more severe and there is a tendency of controlling the use of organic solvents such as isopropyl alcohol from the viewpoint of safety and hygiene. For this reason, it has been desired to develop desensitizing agents or dampening water free of such a compound.
Under such circumstances, Japanese Patent Publication for Opposition Purpose (hereunder referred to as "J. P. KOKOKU") Nos. 55-25075, 55-19757 and 58-5797 disclose compositions containing a variety of surfactants which can only slightly reduce the surface tension of water. In general, the dampening water should have a surface tension ranging from 35 to 50 dynes/cm. Therefore, if these compositions are used as dampening water, it is necessary to substantially increase the concentration of surfactants in such a desensitizing agent or dampening water. Furthermore, water is adhered to an ink film or an ink spreads over the surface of water because of vigorous movement of ink and/or water existing below an ink roll, a printing plate and a roll for supplying dampening water which rotate at a high speed, during the practical lithographic printing. However, combinations of surfactants disclosed in the foregoing methods explained above are insufficient to completely solve these problems. Besides, the dampening water containing such surfactants easily causes foaming during pumping and/or stirring thereof.
In addition, U.S. Pat. No. 3,877,372 discloses a solution containing a mixture of ethylene glycol monobutyl ether and at least one of hexylene glycol and ethylene glycol. U.S. Pat. No. 4,278,467 discloses a dampening water containing at least one member selected from the group consisting of n-hexoxyethylene glycol, n-hexoxydiethylene glycol, 2-ethyl-1,3-hexanediol, n-butoxyethylene glycol acetate, n-butoxydiethylene glycol acetate and 3-butoxy-2-propanol. Japanese Patent Unexamined Publication (hereunder referred to as "J. P. KOKAI")No. 57-199693 (U.S. Pat. No. 4,560,410) discloses dampening water containing 2-ethyl-1,3-hexanediol, Ester diol 204(viz., HOCH.sub.2 C(CH.sub.3).sub.2 CH.sub.3 OCOC(CH.sub.3).sub.2 CH.sub.2 OH), Hexyl Cellosolve or Hexyl Carbitol and at least one member selected from the group consisting of completely water-soluble propylene glycoI, ethylene glycol, dipropylene glycol, diethylene gIycol, hexylene glycol, triethylene glycol, tetraethylene glycol, tripropane glycol and 1,5-pentanediol. As these dampening water compositions do not contain isopropyl alcohol, they are preferable in view of safety and hygiene. However, the wettability thereof with respect to non-image areas of a Iithographic printing plate comprising an anodized aluminum substrate, during printing operation is not sufficient and it is sometimes observed that the non-image areas are contaminated. In particular, during high speed printing operation, so-called ink spreading of half dot image portions, i.e., phenomenon wherein the shape of half dot images is abnormally deformed, is enlarged and is uneven, is caused. Moreover, 2-ethyl-1,3-hexanediol does not have sufficient solubility in water and thus the use thereof is unfavorable to obtain a concentrated dampening water or an additive for dampening water having a high concentration.
These compounds which are substituted for isopropyl alcohol are in general high boiling-point organic solvents. These high boiling-point organic solvents remain, as residues, after the evaporation of the water from dampening water and would attack image areas of PS plates.
Good printing properties can be anticipated through the use of ether type solvents such as ethylene glycol and propylene glycol for lowering the surface tension as well as polymeric compounds such as cellulose derivatives for imparting hydrophilicity and thickening effect to a dampening water composition, but these compounds have a tendency of causing the clouding phenomenon at a temperature of the order of 30.degree. to 50.degree. C. if they are mixed with or dissolved in an aqueous system. In addition, if other components such as salts are admixed, the clouding points of the composition are further lowered and other components dissolved therein are accordingly separated out. This correspondingly becomes an obstacle in concentrating the liquid and lowers the stability of the concentrated liquid with time. Moreover, the resulting concentrated liquid must be used after diluting with a large amount of water to satisfy the requirements stipulated in the Japanese Fire Services Act (content of organic solvent: less than 40%; ignition point: not less than 40.degree. C.; burning point: not less than 60.degree. C.), but particularly the stability of the resulting diluted liquid is often impaired.
The dampening water in general comprises a hydrophilic polymer compound as a component for preventing contamination thereof. If dampening water containing such a hydrophilic polymer is pumped into the dampening water supply device of a printing press through a circulating system, the dampening water overflows from the circulating system due to foaming. For this reason, a silicone type antifoaming agent is generally added to the dampening water. If a concentrated dampening water composition containing a silicone type antifoaming agent is allowed to stand over a long time period (e.g., 1 to 2 months), however, the components of the composition cause separation (liquid-liquid separation) and the composition is liable to cause foaming.